Apparatus for continued operation of an electric motor during an interruption in input power

ABSTRACT

An apparatus is provided for controlling operation of an electric motor through use of an additional power storage arrangement connected across the DC busses of a motor drive and controlling the speed of the motor. The additional power storage arrangement includes an additional capacitor arrangement and a rate limiting arrangement in a series circuit relationship with one another.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This patent application claims the benefit of U.S. Provisional PatentApplication No. 61/524,531, filed Aug. 17, 2011, the disclosure andteachings of which are incorporated herein in their entireties, byreference.

FIELD OF THE INVENTION

This invention generally relates to control and operation of an electricmotor, and more particularly to control and operation of an electricmotor driving a device that has a tendency to be driven backwards atsubstantial speed and/or torque under power disturbance conditions.

BACKGROUND OF THE INVENTION

There are many applications in which a driven device in the form of apump, compressor, etc., is driven at a variable speed by an electricmotor connected to a source of line power through a motor drive. Such amotor drive typically includes an input rectifier section and an outputinverter section. The input rectifier section receives alternatingcurrent power at a constant frequency from the source of line power, andconverts the alternating current received into a DC current supplied ona DC bus at the output of the rectifier section. The inverter sectionhas an input connected to the DC bus, and is controlled to provide analternating current motor drive signal at inverter outputs connected tothe electric motor. The frequency and amplitude of the alternatingcurrent motor drive signal is adjusted as desired to cause the motor torun at a desired motor speed.

Such motor drive arrangements typically also include a power storagearrangement connected across the bus. The power storage arrangementtypically includes a capacitor arrangement, and is configured to becharged and discharged by current flowing into the bus from the inputrectifier section and out of the bus to the inverter section duringoperation of the motor drive in such a manner that the motor drivesignal shape is improved. Another function of the power storagearrangement is to hold the voltage across the bus relatively constantduring minor interruptions or other perturbations of the power beingreceived from the line source.

For many applications, power storage arrangement can be configured toaccommodate even somewhat extended interruptions of power from the linesource by simply increasing the amount of capacitance in the powerstorage arrangement. There are practical limits inherent in this type ofa motor drive arrangement, however, on the degree to which thecapacitance in the power storage arrangement can be increased.

In applications of motor drives where a driven device exhibits a strongtendency to back drive the motor at high speed and/or torque, forexample, the small to moderate capacitance that would typically beincluded in a power storage arrangement of a motor drive may not besufficient to keep the driven device rotating in its normal drivedirection for the full duration of an interruption to the line power.While it would seem to be logical to merely continue to increase thecapacitance in the power storage arrangement to a degree necessary forsustaining continued operation of the motor in driving the driven devicethroughout the duration of the power interruption, there are a number ofpractical limitations and disadvantages to taking this approach.

Motor applications that are susceptible to having the motor be drivenbackwards with substantial speed and/or torque under power disturbanceconditions include applications such as submersible pumps, or pumps andcompressors supplying pressurized fluid to a storage tank located abovethe pump, or a pressurized tank or accumulator.

For example, where the motor is driving a submersible centrifugal pumplocated at the bottom of a wellbore of the type encountered in the oilindustry, there may be a mile or more of fluid in a discharge line abovethe centrifugal pump impeller, between the pump and the surface of theground above. When power to the centrifugal pump is lost through aninterruption of electrical power supplied to the drive motor connectedto the impeller, the weight of fluid in the discharge line above thepump impeller will cause the pump impeller to slow down and eventuallyreverse direction as the fluid in the discharge line is urged bygravitational force to flow backwards through the pump impeller into thewellbore. Where the quantity and weight, or pressure, of the fluid inthe discharge line is substantial, this phenomenon can cause thecentrifugal pump to be accelerated to a very high speed in the reversedirection. Once the impeller begins to rotate in an opposite direction,and particularly once it has begun to rotate at high speed, it may beimpossible for the motor drive to regain control of the motor and returnthe impeller of the pump to its normal rotational direction following arestoration of input power.

If a submersible pump loses power from the mains long enough for thecolumn of fluid to force the motor to spin in reverse, the pump cannotbe restarted until the motor and pump come to rest. It can take a longtime for the column of fluid to descend with the resulting loss ofproduction. Compounding the loss of production problem, it can take along time for the pump to refill the column of fluid after power isrestored before actual production resumes. A large capacitor arrangementattached to the inverter's bus could theoretically keep the driverunning while the mains are interrupted, but the current inrush when themains are restored can destroy the rectifier section of many inverters.Inverters with small bus capacitance do not have the inrush problem, butthe addition of a large capacitor arrangement would recreate theproblem.

In addition to causing undesirably high inrush current, simplyincreasing the capacitance in the power storage arrangement can haveother undesirable consequences. As will be appreciated by those havingskill in the art, with relatively low capacitance in the power storagearrangement, there is a lower level of line harmonics. As thecapacitance in the power storage arrangement is increased to a highlevel, however, the diodes in the rectifier section only conduct at thehigh end of the input voltage, and cause additional amplitude of theline harmonics which are problematic to the utility providing the sourceof line power. Such harmonics can rise to such a level that the utilitywould impose a surcharge for dealing with such harmonics. Further, insome cases where power from an electric utility grid is not available,generators, solar power and/or wind power may be used and such devicesmay be damaged or need to be oversize to deal with the effects ofincreased harmonics.

Another disadvantage of simply increasing the size of capacitors in thepower storage arrangement is that, where the capacitance is relativelysmall, film capacitors can be utilized in the power storage arrangement.Such film capacitors have demonstrated good service life, even whensubjected to significant current ripple resistance. Where a largecapacitance is required, however, film capacitors do not provideadequate energy density and must be replaced by other types ofcapacitors, such as electrolytic capacitors. These capacitor types donot provide some of the desirable inherent characteristics of the filmcapacitors.

What is needed therefore, is an improved apparatus for operation of anelectric motor driving a device that has a tendency to be drivenbackwards with substantial speed and/or torque under power disturbanceconditions.

BRIEF SUMMARY OF THE INVENTION

The invention provides an apparatus and method for controlling theoperation of an electric motor driving a device that has a tendency tobe driven backwards at substantial speed and/or torque under powerdisturbance conditions, through use of a second power storagearrangement connected across the DC bus of a motor drive. The secondpower storage arrangement includes a second capacitor arrangement and arate limiting arrangement connected in a series circuit relationshipwith one another. By virtue of such an arrangement, the capacitance ofthe second capacitor arrangement can be made substantially larger thanthe capacitance of a first power storage arrangement, without incurringdisadvantages of the type discussed above in the background section ofthis application.

In one form of the invention, an apparatus for controlling an electricmotor utilizes a motor drive arrangement including a rectifier section,an inverter section, a first power storage arrangement, and a secondpower storage arrangement. The electric motor is operatively coupled toline power source and rotates in a normal direction. The rectifiersection includes electrical inputs for receiving an input alternatingcurrent line signal, from a line source, and is configured to convertthe input line signal to direct current delivered at plus and minus DCbus outputs of the rectifier section. The inverter section has inputsconnected to the plus and minus DC bus outputs of the rectifier and isconfigured to provide an alternating current motor drive signal atinverter outputs adapted for connection to the electric motor. The firstpower storage arrangement is connected across the plus and minus DC busoutputs of the rectifier section, and has a first capacitor arrangementhaving a first capacitance. The second power storage arrangement is alsoconnected across the plus and minus DC bus outputs of the rectifiersection. The second power storage arrangement includes a secondcapacitor arrangement and a rate limiting arrangement in a seriesrelationship with one another. The capacitance of the second capacitorarrangement is larger than that of the first capacitor arrangement andis configured to provide sufficient power for maintaining the electricmotor rotation, in the normal direction, throughout an interruption inline power.

In some forms of the invention, the second power storage arrangement maybe mounted externally to the remainder of the motor drive arrangement.In one form of the invention, the capacitance of the first capacitor is135 μF and the capacitance of the second capacitor is 0.11 F.

In some forms of the invention, the first capacitor arrangement includesa film capacitor. In various forms of the invention, the secondcapacitor arrangement may include an electrolytic capacitor.

A rate limiting arrangement, according to the invention, may beconfigured to limit the rate at which the second capacitor arrangementcharges from the bus, and be further configured so as to notsignificantly limit the rate at which the second capacitor arrangementdischarges to the bus.

In some forms of the invention, the rate limiting arrangement includes acharging circuit leg, and a discharge circuit leg. The charging circuitleg may have a current limiting resistor arranged in a series circuitrelationship with the second capacitor, to thereby limit current throughthe rate limiting arrangement when the second capacitor arrangement ischarging. The discharge circuit leg may have a discharge diode in aparallel circuit relationship with the charging circuit leg.

In some forms of the invention, the rate limiting arrangement includes acharging circuit leg, and a discharge circuit leg. The charging circuitleg may have a current limiting resistor and a charging diode arrangedin a series circuit relationship with one another and with the secondcapacitor, to thereby limit current through the rate limitingarrangement when the second capacitor arrangement is charging. Thedischarge circuit leg may have a discharge diode in a parallel circuitrelationship with the charging circuit leg.

In some forms of the invention, the rate limiting arrangement mayinclude a charging circuit leg having a buck-type current chopperarrangement comprising a switching device, such as an IGBT, and aninductor in a series circuit relationship with one another and with thesecond capacitor, to thereby limit current through the rate limitingarrangement when the second capacitor arrangement is charging. Thedischarge circuit leg may have a discharge diode in a parallel circuitrelationship with the charging circuit leg including the buck-typecurrent chopper arrangement.

In a motor drive arrangement, according to the invention, a bus voltageis generated across the plus and minus bus connections. The motor drivearrangement may include a controller for controlling the AC motor drivesignal to achieve a desired motor speed of a motor operatively connectedto receive the AC motor drive signal from the inverter. The controllermay further control the AC motor drive signal at least partly as adirect function of the bus voltage, in such a manner that the motorspeed is reduced as function of a reduction in bus voltage. In someforms of the invention, the motor speed may be continually reduced asbus voltage drops.

Those having skill in the art will recognize that by reducing motorspeed as a function of a reduction in bus voltage during an interruptionto line power, the size of components in the second power storagearrangement may be reduced while still providing sufficient power tokeep the motor rotating in its normal direction throughout the durationof an interruption in line power. Conversely, the length of time that agiven configuration of a second power storage arrangement, according tothe invention, will be capable of providing adequate power to the motorto continue rotation in the normal operational direction of the motorwill be increased by reducing the speed of the motor during the powerinterruption condition.

Some forms of the invention may take the form of an apparatus includingeither or both of the electric motor and an input power source.

Other aspects, objects and advantages of the invention will be apparentfrom the following detailed description and accompanying drawingsdescribing exemplary embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings form part of the specification and, togetherwith the written description, serve to explain the principles of theinvention. In the drawings:

FIG. 1 is a schematic illustration of a first exemplary embodiment ofthe invention in the form of a motor and motor drive arrangement,including a motor drive having a second power storage arrangement,according to the invention. Specifically, FIG. 1 illustrates a firstexemplary embodiment of the invention in the form of a test rigincorporating the invention.

FIG. 2 is a graphical illustration of performance of the exemplaryembodiment of FIG. 1 without the addition of the second power storagearrangement, while the test rig is receiving full line power.

FIG. 3 is a graphical illustration of the performance of the firstexemplary embodiment of the invention of FIG. 1, with the addition ofthe second power storage arrangement, operating while receiving fullline power.

FIG. 4 is a graphical illustration of the charging characteristic of acapacitor arrangement in the second storage arrangement, according tothe invention, immediately after the motor drive begins to receive linepower as compared with the charging characteristics of the first storagearrangement.

FIGS. 5-7 are graphical illustrations of the performance of theexemplary embodiment of FIG. 1 during a removal of input power, with themotor being commanded to operate at 3600 RPM in FIG. 5, 2700 RPM in FIG.6 and 1800 RPM in FIG. 7.

FIG. 8 is a graphical illustration of an operational scenario for theinvention, in which the first exemplary embodiment of FIG. 1 is operatedto reduce a speed command to the motor as a function of bus voltageduring an interruption in line power.

While the invention will be described in connection with certainpreferred embodiments, there is no intent to limit it to thoseembodiments. On the contrary, the intent is to cover all alternatives,modifications and equivalents as included within the spirit and scope ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a first exemplary embodiment of the invention in the formof a motor and drive arrangement 100 including a motor drive 102operatively connected to an electric motor 104. Specifically, the firstexemplary embodiment of the motor and drive arrangement 100 illustratesa physical test rig for the invention, in which the electric motor 104was driving a fan to provide a speed dependant load on the electricmotor 104.

As shown in FIG. 1, the exemplary embodiment of the motor drive 102includes a rectifier section 106, an inverter section 108, a first powerstorage arrangement in the form of a first capacitor arrangement 110,and a second power storage arrangement 112.

The rectifier section 106 includes a diode bridge, having electricalinputs R, S, T, for receiving a three-phase alternating current inputline signal from a line source 114. The rectifier section 106 convertsthe input line signals R, S, T to direct current delivered at plus andminus B+, B− DC bus outputs of the rectifier section 106.

The inverter section 108 has inputs connected to the plus and minus busoutputs B+, B− of the rectifier section 106. The inverter section 108provides alternating current drive signals at inverter outputs U, V, Woperatively connected to the electric motor 104.

The first power storage arrangement, in the form of the first capacitorarrangement 110, is connected across the plus and minus bus outputs B+,B− and has a first capacitance. As indicated in FIG. 1, the firstcapacitance in the exemplary embodiment 100 is 135 μF. It should beunderstood that the total first capacitance can be provided by more thanone capacitor having ratings appropriate for the intended use.

The second power storage arrangement 112 is also operatively connectedacross the plus and minus bus outputs B+, B−. The second power storagearrangement 112 includes a second capacitor arrangement 116 and a ratelimiting arrangement 118 disposed in a series circuit relationship withone another. The capacitance of the second capacitor arrangement 116 islarger than the first capacitance of the first capacitor arrangement110. Specifically, in the exemplary embodiment shown in FIG. 1, thecapacitance of the second capacitor is 0.11 F. It should be understoodthat the total second capacitance can be provided by more than onecapacitor having ratings appropriate for the intended use.

As illustrated in FIG. 1, the second power storage arrangement 112 maybe physically located outside of the remainder of the motor drivearrangement 102. Externally locating the second power storagearrangement may be preferable where a large amount of capacitance isrequired for the second capacitor arrangement 116, in order to allow theremainder of the motor drive to be contained in a smaller volume. Thosehaving skill in the art will recognize, however, that the second powerstorage arrangement need not be located physically outside of theremainder of the motor drive in practicing the invention.

As further illustrated in the exemplary embodiment of FIG. 1, the ratelimiting arrangement 118 in the exemplary embodiment includes a chargingcircuit leg including a current limiting resistor 120 and a chargingdiode 122 arranged in a series circuit relationship with one another andalso with the second capacitor arrangement 116. The charging diode 122is oriented to allow current flow through the charging circuit leg andthe current limiting resistor 120 to thereby limit the rate at which thesecond capacitor arrangement 116 can charge when the motor drive 102 isreceiving power from the line source 114.

The rate limiting arrangement 118 in the exemplary embodiment of themotor drive 102 shown in FIG. 1 also includes a discharge circuit leg inthe form of a discharge diode 124 connected in a parallel circuitrelationship with the current limiting resistor 120 and charging diode122 forming the charging circuit leg. The discharge diode 124 isoriented to bypass the current limiting resistor 120 when the secondcapacitor arrangement 116 is discharging during periods of operationwhere the line voltage supplied by the source of line power 114 isabsent or has drooped to a level lower than the voltage at which thefirst and second capacitor arrangements 110, 116 were charged.

In some forms of the invention, a controller 126 may reduce the speed ofmotor 104 to a predetermined lower value upon detection of a thresholddrop in voltage across the bus as measured at the terminals B+ and B−.In other forms of the invention, the controller 126 may reduce the speedof motor 104 as a function of the bus voltage. Such a function may be aproportionality, or any other desired function between the sensed busvoltage and the motor speed. The controller 126 is coupled to theinverter 108 at the input of the inverter section 108 and configured toprovide commands to the inverter section 108, for example a reducedmotion velocity command.

Operation of the test rig shown in the exemplary embodiment of FIG. 1will now be described with reference to a series of graphicalrepresentations of performance of the exemplary embodiment 100 invarious configurations and operational modes.

The component values shown in FIG. 1 were used for a test setup. The0.11 F second capacitor arrangement 116, in conjunction with 25 ohmresistor 120 and diode 122 limit the current inrush to 26 amps (ratedcurrent at full load). The time constant is 2.75 seconds so it takesabout 15 seconds to charge the second capacitor arrangement 116. Thislimits the repetition rate of mains interrupts. The reverse diode 124allows any amount of current to aid the D.C. bus—during a mains outage.

For analysis and testing, the 3600 RPM 30 HP motor 104 had a truck fanattached to provide a load. Table 1 shows the power used by the fan atvarious speeds. The power closely follows a cube law vs. RPM.

TABLE 1 RPM Motor Power Motor Volts 1800 3.5 HP, 2.6 KW 270 2700 10.6HP, 7.9 KW  397 3600 22.0 HP, 16.4 KW 456

The test results for line current 200 and the bus voltage 202 shown inFIGS. 2 and 3, and tabulated in Table 2, demonstrate that when theinverter 108 is running, the diodes 122, 124, resistor 120, and secondcapacitor arrangement 116 does not significantly change performance ofthe motor drive 102 or the system from what they were prior to additionof the second power storage arrangement 112.

TABLE 2 Harmonics in Percent Amps THD 1^(st) 3^(rd) 5^(th) 7^(th) 9^(th)11^(th) 13^(th) 15^(th) S1200 w/o Cap 24 47% 88 — 21 18 — 15 12Arrangement S1200 with Cap 24 42% 90 — 22 15 — 4.5 2.4 Arrangement

The harmonics and Total Harmonic Distortion (THD) comparison for themotor drive 102 and system 100 with and without addition of the secondpower storage arrangement 112 is about the same with the differencesbeing within the measurement error.

As shown in FIG. 4, if the Thevenin model of the mains 114 has lowseries impedance, then the first power storage arrangement 110 and thesecond power storage arrangement 112 will charge as if they have theirown separate source. The resistor 120 in the second power storagearrangement 112 has no damping effect on the remainder of the circuitinside the motor drive 102. FIG. 4 shows an overshoot of the bus voltage202 to 1000 volts when the mains 114 were switched on. In theory itcould have been as high as 1300 volts. The charging characteristic ofthe second capacitor arrangement 116 is shown by the second powerstorage arrangement voltage 204.

The time scale on the second power storage arrangement voltage 204, inFIG. 4, is too fast to show the exponential charging curve. The 0.11 Fcapacitor arrangement 116 in the test circuit stores 19.8 KJ and thesame amount of energy is dissipated in the 25 ohm resistor 120 duringcharging. The resistor 120 used for the test is rated 1.5 KW and theinitial power when the mains are first engaged is 14.4 KW, so theresistor 120 must dissipate a significant amount of heat. Carefulresistor selection is required for a working system. A “buck-type”current chopper comprising a semiconductor switch, such as an IGBT, andan inductor could be substituted for the resistor 120 and diode 122.

FIGS. 5-7 show the peak motor voltage 206 applied to motor 104 and busvoltage 202 at various motor speeds when the mains 114 are lost. Each ofFIGS. 5-7 show that the bus voltage 202 is 600 volts while the motor isrunning with the mains 114 switched on and that the bus voltage 202 hasdecreased to 350 volts when the controller 126 shuts off inverter 108 atthe low bus voltage threshold. The capacitor arrangement 116 hasdelivered ½*0.11 F*(600²−350²)=13 KJ during the period 210 after themains 114 are lost and prior to the controller 126 shutting off theinverter 108.

In the operating scenarios shown in FIGS. 5-7, the motor 104 is stillcommanded to run at normal operating speed, i.e. 3600 RPM in FIG. 5,2700 RPM in FIG. 6, and 1800 RPM in FIG. 7 as the bus voltage decaysafter mains 114 are lost. The supporting peak motor voltage 206 isdropping and the motor current waveform is deteriorating.

FIG. 5 shows the motor drive 102 running 3600 RPM delivering 16.4 KW tomotor 104 as the mains 114 open. It takes 0.6 seconds for controller 126to shut off inverter 108 at the low bus voltage threshold of 350V so arough energy estimate is 16.4 KW*0.6 seconds=9.8 J. This is similar tothe 13 KJ prediction from the stored energy in the capacitorarrangements 110 and 116.

As shown in FIG. 6, when the test is repeated with the motor drive 102running motor 104 at 2700 RPM while delivering 7.9 KW, it takes 1.6seconds for controller 126 to shut off inverter 108 at the low busvoltage threshold of 350V (1.6 sec*7.9 KW=12.6 KJ).

As shown in FIG. 7, at 1800 RPM at 2.66 KW, the motor drive 102 runs for4.2 seconds before controller 126 shuts off inverter 108 at the low busvoltage threshold of 350V (4.2 sec*2.66 KW=11.2 KJ).

As shown in FIG. 5, with the motor drive 102 continuing to command themotor 104 to run at 3600 RPM while the bus is discharging, the currentwaveforms cannot continue to deliver optimal power during this time andthe drive trips within 0.6 seconds. In the operational scenario shown inFIG. 8, a change is made to the commands that controller 126 provides toinverter 108 such that the velocity command to the motor 104 is adjustedproportionally to the bus voltage 202 as the bus voltage 202 isdropping. The gain is set such that when the bus is 600 VDC or greaterthe velocity command is 3600 RPM. When the mains 114 are turned off, thevelocity command diminishes with the bus voltage 202 until the lowvoltage bus trip is reached.

The bus voltage 202 and the second power storage arrangement voltage 204have their origin at the bottom left corner of FIG. 8. The peak motorarmature current 208 is zeroed at the center of FIG. 8.

When the mains 114 are turned off, the drive 102 continues to run for2.6 seconds before the low voltage trip. This is a factor of 4.3 timeslonger than the 0.6 seconds when the velocity command was fixed at 3600RPM, in the operational scenario shown in FIG. 5.

Those having skill in the art will recognize that the values of circuitcomponents in the exemplary embodiment 100 described herein are merelyillustrative and not limiting with regard to practice of the invention.In other forms of the invention, component values would likely bedifferent.

The use of the terms “a” and “an” and “the” and similar referents in thecontext of describing the invention (especially in the context of thefollowing claims) is to be construed to cover both the singular and theplural, unless otherwise indicated herein or clearly contradicted bycontext. The terms “comprising,” “having,” “including,” and “containing”are to be construed as open-ended terms (i.e., meaning “including, butnot limited to,”) unless otherwise noted. Recitation of ranges of valuesherein are merely intended to serve as a shorthand method of referringindividually to each separate value falling within the range, unlessotherwise indicated herein, and each separate value is incorporated intothe specification as if it were individually recited herein. All methodsdescribed herein can be performed in any suitable order unless otherwiseindicated herein or otherwise clearly contradicted by context. The useof any and all examples, or exemplary language (e.g., “such as”)provided herein, is intended merely to better illuminate the inventionand does not pose a limitation on the scope of the invention unlessotherwise claimed. No language in the specification should be construedas indicating any non-claimed element as essential to the practice ofthe invention.

Preferred embodiments of this invention are described herein, includingthe best mode known to the inventors for carrying out the invention.Variations of those preferred embodiments may become apparent to thoseof ordinary skill in the art upon reading the foregoing description. Theinventors expect skilled artisans to employ such variations asappropriate, and the inventors intend for the invention to be practicedotherwise than as specifically described herein. Accordingly, thisinvention includes all modifications and equivalents of the subjectmatter recited in the claims appended hereto as permitted by applicablelaw. Moreover, any combination of the above-described elements in allpossible variations thereof is encompassed by the invention unlessotherwise indicated herein or otherwise clearly contradicted by context.

What is claimed is:
 1. An apparatus for controlling an electric motorthrough use of a motor drive apparatus, the electric motor operativelycoupled to line power and rotating in a normal direction, the apparatuscomprising: a rectifier section having electrical inputs for receivingan input alternating current line signal from a line source and plus andminus bus outputs for providing a direct current bus signal; an invertersection having inputs connected to the plus and minus bus outputs of therectifier section and providing an alternating current motor drivesignal at inverter outputs adapted for connection to the electric motor;a first power storage arrangement connected across the plus and minusbus outputs and having a first capacitor arrangement having a firstcapacitance; and a second power storage arrangement connected across theplus and minus bus outputs and having a second capacitor arrangementhaving a second capacitance and a rate limiting arrangement in a seriesrelationship with the second capacitor arrangement, with the secondcapacitance larger than the first capacitance and configured to providesufficient power for maintaining the electric motor rotation, in thenormal direction, throughout an interruption of power.
 2. The apparatusof claim 1, including the rate limiting arrangement comprising a diodeand a resistor configured to limit charging rate of the second capacitorarrangement.
 3. The apparatus of claim 2, including the rate limitingarrangement further comprising a second diode configured to allow thevoltage of the second capacitor arrangement to provide voltage to theinverter section.
 4. The apparatus of claim 3, wherein the firstcapacitor arrangement comprises a film capacitor and the secondcapacitor arrangement comprises an electrolytic capacitor.
 5. Theapparatus of claim 4, further comprising a controller configured tosense a voltage at the inputs of the inverter section and provide areduced motor velocity command to the inverter section when a reducedvoltage is sensed at the inputs of the inverter section when compared toa nominal value of voltage at the inputs of the inverter section.
 6. Theapparatus of claim 5, wherein the reduced motor velocity command islimited to a maximum value that is calculated by multiplying a normaloperating speed of the electric motor connected to the apparatus by thereduced voltage sensed at the inputs of the inverter section and thendividing by the nominal value of voltage at the inputs of the invertersection.
 7. The apparatus of claim 3, further comprising a controllerconfigured to sense a voltage at the inputs of the inverter section andprovide a reduced motor velocity command to the inverter section when areduced voltage is sensed at the inputs of the inverter section whencompared to a nominal value of voltage at the inputs of the invertersection.
 8. The apparatus of claim 7, wherein the reduced motor velocitycommand is limited to a maximum value that is calculated by multiplyinga normal operating speed of the electric motor connected to theapparatus by the reduced voltage sensed at the inputs of the invertersection and then dividing by the nominal value of voltage at the inputsof the inverter section.
 9. The apparatus of claim 1, including the ratelimiting arrangement comprising a semiconductor switch and an inductorconfigured to limit charging rate of the second capacitor arrangement.10. The apparatus of claim 9, including the rate limiting arrangementfurther comprising a diode configured to allow the voltage of the secondcapacitor arrangement to provide voltage to the inverter section. 11.The apparatus of claim 10, wherein the first capacitor arrangementcomprises a film capacitor and the second capacitor arrangementcomprises an electrolytic capacitor.
 12. The apparatus of claim 11,further comprising a controller configured to sense a voltage at theinputs of the inverter section and provide a reduced motor velocitycommand to the inverter section when a reduced voltage is sensed at theinputs of the inverter section when compared to a nominal value ofvoltage at the inputs of the inverter section.
 13. The apparatus ofclaim 12, wherein the reduced motor velocity command is limited to amaximum value that is calculated by multiplying a normal operating speedof the electric motor connected to the apparatus by the reduced voltagesensed at the inputs of the inverter section and then dividing by thenominal value of voltage at the inputs of the inverter section.
 14. Theapparatus of claim 10, further comprising a controller configured tosense a voltage at the inputs of the inverter section and provide areduced motor velocity command to the inverter section when a reducedvoltage is sensed at the inputs of the inverter section when compared toa nominal value of voltage at the inputs of the inverter section. 15.The apparatus of claim 14, wherein the reduced motor velocity command islimited to a maximum value that is calculated by multiplying a normaloperating speed of the electric motor connected to the apparatus by thereduced voltage sensed at the inputs of the inverter section and thendividing by the nominal value of voltage at the inputs of the invertersection.
 16. The apparatus of claim 1, wherein the first capacitorarrangement comprises a film capacitor.
 17. The apparatus of claim 1,wherein the second capacitor arrangement comprises an electrolyticcapacitor.
 18. The apparatus of claim 1, further comprising a controllerconfigured to sense a voltage at the inputs of the inverter section andprovide a reduced motor velocity command to the inverter section when areduced voltage is sensed at the inputs of the inverter section whencompared to a nominal value of voltage at the inputs of the invertersection.
 19. The apparatus of claim 18, wherein the reduced motorvelocity command is limited to a maximum value that is calculated bymultiplying a normal operating speed of the electric motor connected tothe apparatus by the reduced voltage sensed at the inputs of theinverter section and then dividing by the nominal value of voltage atthe inputs of the inverter section.
 20. A method for controlling anelectric motor with a motor drive apparatus, the electric motoroperatively coupled to a line power source and rotating in a normaldirection, the method comprising: coupling a rectifier sectionconfigured for receiving an input alternating current line signal fromthe line power source for providing a direct current bus signal;coupling an inverter section to the plus and minus bus outputs of therectifier section and providing an alternating current motor drivesignal at inverter outputs configured to connect to the electric motor;connecting a first power storage arrangement having a first capacitorarrangement including a first capacitance across the plus and minus busoutputs and; connecting a second power storage arrangement having asecond capacitor arrangement including a second capacitance and a ratelimiting arrangement in a series relationship with the second capacitorarrangement across the plus and minus bus outputs, wherein the secondcapacitance is larger than the first capacitance; and coupling acontroller to the inverter section and the bus outputs of the rectifiersection, the controller configured to sense a voltage at the inputs ofthe inverter section and provide a reduced motor velocity command to theinverter section when a reduced voltage is sensed at the inputs of theinverter section when compared to a nominal value of voltage at theinputs of the inverter section, with the reduced motor velocity commandlimited to a maximum value that is calculated by multiplying a normaloperating speed of the electric motor connected to the apparatus by thereduced voltage sensed at the inputs of the inverter section and thendividing by the nominal value of voltage at the inputs of the invertersection.